Interestingly, the immune system becomes stronger when it is fighting a disease that it has already encountered before. So during vaccination, the body’s immune system is injected with the certain type of virus or bacteria in order to introduce the body to the disease and help the system combat it and get stronger when battling against the real one. For example, in order to gain immunity to small pox, a vial containing the disease is injected to a child so that he cannot catch the disease in the future.